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HT
2
NN
2
LN
0
CT
3
MH
4 tháng 2 2016
-210 = (-1) + (-2) + (-3) + ... + (-x + 1) + (-x)
=> -210 = -(1 + 2 + 3 + ...+ x - 1 + x)
=> 210 = 1 + 2 + 3 + x - 1 + x
=> 210 = \(\frac{\left(x+1\right).x}{2}\)
=> 420 = (x + 1).x
=> 21 . 20 = (x + 1) . x
=> (20 + 1) . 20 = (x + 1) . x
=> x = 20
25 tháng 1 2017
- hgvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
HT
1
14 tháng 2 2016
Ta có: -210=(-1)+(-2)+(-3)+...+(-x+1)+(-x)
-210=-[1+2+3+...+(x-1)+x)]
-210=\(-\left[\frac{x\left(x+1\right)}{2}\right]\)
hay 210=\(\left[\frac{x\left(x+1\right)}{2}\right]\)
=> x(x+1)=210.2=420
=> 20.21=420 => x=20
NS
2
L
2
HL
0
DT
0
a) (3x -2)^5 = -243
(3x -2)^5 = (-3)^5
3x-2=-3
3x=-1
x=-1/3
b) ( x-7)^( x+1) - ( x-7)^(x+11) = 0
( x-7)^( x+1) - ( x-7)^(x+1)*x^10 = 0
( x-7)^( x+1) (1-x^10) = 0
Do x phải là số nguyên tố nên x=7
sai rùi